1. Field of the Invention
This invention relates to a four wheel driving vehicle of a front/rear wheel differential operation limiting type. More particularly, it relates to a four wheel driving vehicle with a front/rear wheel torque distribution controlling system of the differential limiting type which limits a differential operation condition between the front wheel side and the rear wheel side to control a torque distribution between the front wheels and the rear wheels.
2. Description of the Prior Art
Various power transmitting apparatus for an automobile are already known which control a ratio between a torque transmitted to the front wheel side and another torque transmitted to the rear wheel side in response to an operating condition of the automobile.
For example, it may be a possible construction, for example, of a so-called full time four wheel driving automobile that, in order to suitably distribute a driving force of an engine between the front wheel side and the rear wheel side, a center differential gear and a differential operation limiting mechanism such as a viscous coupling for limiting a differential operation of the center differential gear are provided, and the differential limiting mechanism is adjusted to control the ratio in torque in response to an operating condition of an automobile.
Particularly, as a controlling device to cope with a case when a spin takes place at the four wheels, various means have been proposed including means for controlling a torque distribution between the front and rear wheels in response to a lateral acceleration acting upon the vehicle body or a vehicle speed and means for distributing a torque between the front and rear wheels such that it may increase simply in primary proportion to an advancing direction acceleration acting upon the vehicle body.
However, even if the torque distribution between the front and rear wheels is controlled in response to a lateral acceleration or a vehicle body speed or otherwise controlled such that it may increase simply in primary proportion to an advancing direction acceleration as described just above, an optimum torque distribution cannot be achieved.
Also such a method has been proposed that, taking durability of a driving system into consideration, a differential operation limiting force is maintained at a predetermined value when an input torque from an engine is higher than a predetermined value. While such method assures durability of the driving system, the performance in operation is not improved.
It may also be a possible method in a full time four wheel driving automobile that a differential operation limiting clutch for limiting a differential operation of a center differential gear is provided so that the torque ratio may be controlled in response to an operating condition of the automobile by way of the differential limiting clutch.
Here, in order to assure a high responsibility of such differential operation limiting clutch as described above, normal solution would be to apply, taking into consideration an abrasion of the clutch, a dispersion of springs and so forth, a hydraulic pressure higher than an engagement starting hydraulic pressure to the clutch to maintain the clutch in a slightly engaged condition. However, while the clutch is new, it has an excessively high drag. Further, as abrasion of the clutch proceeds, a gap will form between clutch elements when control of the clutch is not necessary, which has a bad influence on fuel cost and performance of the engine.